1. Field of the Invention
The present invention relates to polishing, and more particularly to a polishing pad and a method for polishing semiconductor wafers.
2. Description of Related Art
In the manufacture of integrated circuits, the planarization of semiconductor wafers is becoming increasingly important as the number of layers used to form integrated circuits increases. For instance, metallization layers formed to provide interconnects between various devices may result in nonuniform surfaces. The surface nonuniformities may interfere with the optical resolution of subsequent lithographic steps, leading to difficulty with printing high resolution patterns. The surface nonuniformities may also interfere with step coverage of subsequently deposited metal layers and possibly cause open or shorted circuits.
Various techniques have been developed to planarize the top surface of a semiconductor wafer. One such approach involves polishing the wafer using a polishing slurry that includes abrasive particles mixed in a suspension agent. With this approach, the wafer is mounted in a wafer holder, a polishing pad has its polishing surface coated with the slurry, the pad and the wafer are rotated such that the wafer provides a planetary motion with respect to the pad, and the polishing surface is pressed against an exposed surface of the wafer. The polishing erodes the wafer surface, and the process continues until the wafer is largely flattened. Typically, the slurry is introduced near the center of the pad, forms a ring around the wafer and goes under the wafer as necessary. It is generally desirable to maintain an adequate amount of slurry between the wafer and the pad while dispensing as little slurry as possible to lower costs.
In chemical-mechanical polishing, the slurry particles abrade the wafer surface while a chemical reaction occurs at the wafer surface. For instance, in chemical-mechanical polishing of silicon dioxide, the slurry particles generate high pressure areas that cause the silicon dioxide to react with water. In chemical-mechanical polishing of other materials, such as tungsten, the slurry employs a wet chemical etchant to assist in removing wafer material. The wet chemical etchant is often more selective to the exposed wafer material than to underlying wafer materials.
The polishing pad can be a felt fiber fabric impregnated with polyurethane, with the amount of impregnation determining whether the pad is a "hard pad" or a "soft pad." A hard pad tends to focus the polishing pressure on protuding regions of the wafer surface in order to rapidly planarize the wafer surface. A soft pad tends to create a more even polish over the entire wafer surface, a finer surface finish, and less mechanical damage to the wafer.
Polishing pads with various topographies that improve the polishing operation are known in the art. In particular, polishing pads have been designed with channels, voids and the like in the polishing surface for reducing radially-dependent variations in the polishing rate. For instance, polishing pads may include voids that reduce radially-dependent variations in the surface contact rate. Alternatively, polishing pads may include circumferential or radial grooves that reduce radially-dependent variations in the slurry flow. The following are some examples.
U.S. Pat. No. 5,020,283 discloses a polishing pad containing circular voids in which the voids are substantially the same size but the frequency of voids increases with increasing radial distance. U.S. Pat. No. 5,177,908 discloses a polishing pad containing a sunburst pattern of nontapered rays, a polishing pad containing orthogonal channels in which the distance between channels decreases with increasing radius, and a polishing pad containing voids in which the void size increases with increasing radius. U.S. Pat. No. 5,394,655 discloses a polishing pad having a segmented circumferential strip near the outer circumferential edge, and another segmented circumferential strip near an inner circumferential edge, such that each circumferential strip encounters the edge of a wafer moved cycloidally with respect to the pad.
In other polishing pads, the polishing surface may include a series of circumferential grooves that direct the slurry between the pad and the wafer in order to prevent hydroplaning. These grooves are usually formed only on the portion of the polishing surface which contacts the wafer. U.S. Pat. No. 5,216,843 observes that circumferential macrogrooves become worn down over time. To alleviate this problem, the '843 patent utilizes a polishing apparatus that continually conditions the polishing pad by forming radial microgrooves in the pad while polishing occurs. The apparatus includes a diamond block holder with embedded diamond tipped threaded shanks that generate the microgrooves as a holder block is swept across the pad surface during polishing. The microgrooves are interconnected to one another and are 40 microns deep. There are several drawbacks to this approach. First, the conditioning apparatus requires special gearing and design to perform optimally. Furthermore, since the microgrooves have very small, uniform depths and widths, a significant amount of slurry can build up around the edges of the wafer and/or flow past the wafer and be wasted.
Accordingly, a need exists for a polishing pad and method of polishing which provides improved control over slurry and other fluids during polishing.